1. Field of the Invention
The present invention relates to an energy filter for selectively passing charged particles having certain energies and to an electron microscope using this filter.
2. Description of Related Art
In recent years, a transmission electron microscope has been developed which has an energy filter positioned within the electron optical system for magnifying and projecting an electron beam transmitted through a specimen onto a fluorescent screen. With this transmission electron microscope fitted with such an energy filter, an electron microscope image of a specimen can be formed based on charged particles having only certain energies.
In this electron microscope fitted with such an energy filter, the energy width of the electron beam directed at the specimen must be narrowed in order to enhance the energy resolution. For example, in an electron microscope using an accelerating voltage of hundreds of kilovolts, the energy width is preferably on the order of 0.1 eV. The electron beam passed through the energy filter in this way is required to be achromatic, i.e., free of energy dispersion.
Therefore, a method consisting of placing an energy filter in a position where an electron beam is not yet accelerated and assumes a low energy state, performing energy selection, and then accelerating the beam to give high energy to it has been developed. The energy filter that provides energy selection in this way is known as a monochromator.
Where an electron beam with low energy passes through an energy filter, the influences of the Boersch effect must be taken into account. The Boersch effect is a phenomenon in which the energies of electrons (generally, charged particles) traveling close to each other affect each other according to their respective speeds due to their mutual Coulomb interactions. Accordingly, where an electron beam of low energy passes through the energy filter, especially when electrons travel close to each other around the focal point within the filter, the electrons affect each other, varying their energies. This broadens their energies. Consequently, it is difficult to obtain an electron beam having a narrow energy width.
The sole method conventionally adopted such that the energy filter placed in a position where the electron beam is not yet accelerated is free from the Boersch effect is to prevent crossover of beam electrons by astigmatic focusing.
FIGS. 8(a)-8(d) show the structure of a first energy filter that forms an astigmatic image. This and other figures are not drawn to scale, for convenience of illustration of the first energy filter 110. As shown in FIG. 8(a), the first energy filter 110 has two stages of Wien filters 112 and 114 that extend in the direction of the optical axis L0, or the Z-direction. The filters 112 and 114 are located before and after the position of a slit 113 asymmetrically with respect to this position (see Patent Reference 1 (Japanese Patent Laid-Open No. 2001-23558), for example). In particular, the first energy filter 110 has a first electrostatic lens 111, the first stage of Wien filter 112, the slit 113, the second stage of Wien filter 114, and a second electrostatic lens 115 in this order as viewed along the direction of the electron beam that is emitted from an electron gun 201 and travels in the Z-direction.
In the first energy filter 110, the electron beam takes an orbit (FIG. 8(b)) along the X-axis that is the dispersive direction, an orbit (FIG. 8(c)) extending in the Y-direction perpendicularly to that orbit, and a dispersive orbit (FIG. 8(d)). As can be seen from these orbits, the X-direction electron beam is focused at the position of the slit 113 but the Y-direction beam is not.
FIG. 9 shows the profile portions of the electron beam focused on the slit 113 of the first energy filter. The beam focused on the slit 113 assumes an elliptical form extending in the Y-direction. Because the first energy filter 110 has a dispersive direction in the X-direction, beam electrons having different energies are focused at different positions in the X-direction. The profile portions 117 and 118 of the beam electrons having different energies and focused at different positions in the X-direction are shown in the figure.
In the first energy filter 110, the position at which the electron beam is focused can be adjusted in the Y-direction. Therefore, the ratio between the width W and height H of the profile of the electron beam on the slit 113 can be adjusted.
However, in the first energy filter 110, acceleration performed behind the second stage of Wien filter 114 makes it impossible to eliminate the effects of aberrations created by the first and second stages of Wien filters 112 and 114 at the focal point of the electron beam behind the accelerating electrode. The final profile of the electron beam contains aberration.
FIGS. 10(a) to 10(b) show the structure of a second energy filter. As shown in FIG. 10(a), the second energy filter, indicated by 120, has two stages of Wien filters 122 and 124 that extend in the direction of the optical axis L0, or the Z-direction. The filters 122 and 124 are located before and after the position of a slit 123 symmetrically with respect to this position. In particular, the second energy filter 120 has a first electrostatic lens 121, the first stage of Wien filter 122, the slit 123, the second stage of Wien filter 124, and a second electrostatic lens 125 in this order as viewed along the direction of the electron beam that is emitted from an electron gun 201 and travels in the Z-direction. The filters 122 and 124 are symmetrical in shape with respect to the slit 123.
In the second energy filter 120, the electron beam takes an orbit (FIG. 10(b)) in the X-direction that is the dispersive direction and an orbit (FIG. 10(c)) extending in the Y-direction perpendicularly to that orbit. As can be seen from these orbits, the orbits of the electron beam through the Wien filters 122 and 124 are symmetrical with respect to the position of the slit 123. Specifically, the orbit of the beam is inverted with respect to the focal position on the slit 123. Where the orbit has such symmetry, the aberrations in the first and second stages of Wien filters 122 and 124 cancel each other at the focal point of the electron beam behind the second stage of Wien filter 124, resulting in a beam with reduced aberration. FIG. 10(d) shows the dispersive orbit of the electron beam.
Also, in the second energy filter 120, the profile of the electron beam on the slit 123 is such that the height is greater than the width in the same way as in the case of the first energy filter 110. In the configuration of this second energy filter 120, however, the ratio of the width to the height of the profile of the electron beam on the slit 123 is determined only by the incident angle of the electron beam. To prevent this ratio from becoming excessive, a well-collimated beam must be entered through a small aperture.
Where the profile of the electron beam is elongated in the Y-direction by focusing the beam on the slit in the X-direction (along the width) by the aforementioned astigmatic focusing and unfocusing the beam in the Y-direction (along the height), the ratio of the width to the height becomes excessively large. Consequently, when energy selection is performed on the slit, the electron beam will produce greater loss or the energy width will not be reduced as expected.
The purpose of elongating the profile of the electron beam is to reduce Coulomb interactions, such as the Boersch effect. Therefore, the profile of the beam is not elongated too much. It is necessary to set the ratio to 1:2 or higher. However, a large ratio, such as 1:10, is not necessary. The ratio may be set to intermediate values between them.
In the case of an energy filter having Wien filters that are located ahead of and behind the slit 113 asymmetrically with respect to this slit as in the first energy filter 110 already described in connection with FIGS. 8(a)-8(d), the profile of the electron beam at the slit 113 can be set to a value within the range described above. However, the first energy filter 110 cannot cancel aberrations by the first and second stages of Wien filters 112 and 114 as mentioned previously. In consequence, the electron beam going out of the first energy filter 110 contains aberration.
Of course, the aberration in the energy filter can be reduced by well designing the first stage of filter, as well as by canceling out the aberrations in the first and second stages of Wien filters 122 and 124 that are symmetrical with respect to the slit 123 as shown in the second energy filter 120.
FIG. 11 shows a Wien filter with small aberration. This Wien filter, indicated by 130, is designed to reduce the aberration by bringing the electric field distribution in the fringing field into coincidence with the magnetic field distribution. This in turn is achieved by producing electric and magnetic fields using eight poles P1 to P8 (see, for example, Patent Reference 2 (Japanese Patent No. 3,040,245)). Also shown in the figure are coils C1 to C8. Aberration still remains, however, even if this Wien filter having the eight poles is used. Hence, there is a demand for a technique of reducing this aberration.